The invention relates to an optical information carrier for inscribing, reading and erasing information by means of a laser-light beam, said optical information carrier comprising a transparent substrate having a servo track and a stack of layers, which stack comprises at least one recording layer of a phase-change material and at least one reflection layer of metal.
Known optical information careers, such as the Compact Disc (CD), Compact Disc Read-Only-Memory (CD-ROM) and Laser Vision (LV) are provided with information by the manufacturer and can only be read by the user by means of commercially available playback equipment. Other information carriers can be recorded once (CD-R) by the user. A information carrier which is recordable and erasable by the user and which is readable in a standard CD player could be used for many applications. A special recording/erase device is used for recording and erasing. Audio or data information can be recorded in the information carrier by the user himself. After an erasing step, new information can be recorded in the information carrier.
A prerequisite for reading a recordable and erasable information carrier in a standard CD player is that said information carrier is compatible with said CD player, i.e. it must comply with the industrial standard for CD systems, hereinafter referred to as CD-industrial standard. This means, among other things, that the reflection R.sub.H of unrecorded areas of the information carrier and the modulation must have a specific minimum value. When a recorded information carrier is read by means of a focused laser-light beam, reflection differences and/or optical path-length differences bring about a modulated laser-light beam which is subsequently converted by a detector into a modulated photocurrent in accordance with the coded, recorded digital information. The modulated photocurrent is a HF signal whose lowest fundamental frequency is 196 kHz. The peak-to-peak value of the photocurrent is designated I.sub.11 and the top level of the HF signal associated with 196 kHz is designated I.sub.top. The modulation is defined as I.sub.11 /I.sub.top and must be at least 0.6 according to the above-mentioned CD-industrial standard. The modulated photocurrent is caused by reflection differences and/or optical path-length differences between recorded and unrecorded areas of the information carrier. Information carriers whose modulation is based only on reflection differences must have a minimal optical contrast in order to attain the required modulation of 0.6. The optical contrast C is defined as: EQU C=100(R.sub.H -R.sub.L)/R.sub.H
In this equation, R.sub.H has the above-mentioned meaning and R.sub.L is the reflection of recorded areas (bits) of the information carrier. In practice, the modulation is lower than the optical contrast due to the size of the laser spot. This means that, in order to meet the modulation requirement of 0.6, the optical contrast C must be higher than 60%. The reflection requirement R.sub.H of unrecorded areas is based on a laser wavelength of 780 nm.+-.10 nm. For a perpendicularly incident, parallel laser-light beam of the above wavelength, the reflection of the substrate surface (approximately 4%) also being measured, R.sub.H must be at least 70%. In the case of a focused laser-light beam, as used in a CD player, a reflection value R.sub.H of minimally 65% is sufficient, i.e. such a information carrier can be played in a standard CD player and is compatible therewith.
A known type of optical information carder has a recording layer of a so-called phase-change material. By locally heating said recording layer with a focused laser-light beam and subsequently cooling it, the state of crystallization of these materials is changed at the location of the laser spot and a readable bit is formed. Dependent upon the material of the recording layer, amorphous material changes to crystalline material or conversely. Conversion of a crystalline phase to another crystalline phase is also possible. The recording layer is, for example, crystalline and has the property that incident laser light is absorbed. During recording information, the information carrier is rotated and exposed to a focused laser-light beam which is modulated in accordance with the information to be inscribed. By virtue thereof, amorphous information bits are formed in the exposed areas of the recording layer which remains crystalline in the unexposed areas. The crystalline material has other optical properties than the amorphous material, so that the recorded information can be read in reflection as an optical contrast by means of a low-power, focused laser-light beam. In some phase-change materials, the amorphous areas can be reconvened into crystalline areas, or conversely, by heating, so that the recorded information is erased. Subsequently, new information can again be recorded in the recording layer.
It has been found to be very difficult to manufacture a recordable information career which meets the requirements of the so-called CD-industrial standard. The reflection and contrast of the known phase-change materials are too low, so that, without further treatment, said materials are unsuitable for use in a information carrier which is compatible with the CD player.
An information carrier of the type mentioned in the opening paragraph is known from European Patent Application EP-A-352105 to which U.S. Pat. No. 5,144,618 corresponds. The known information carrier of the phase-change type consists of a substrate which is provided with a spiral-shaped servo track which carries a stack of layers consisting of a recording layer of an InSb or TeGe alloy, a dielectric layer of, for example, ZnS and a reflection layer of, for example, Au. In a preferred embodiment of the known information carrier, the reflections amount to 72% and 38% for the unrecorded and the recorded areas, respectively. Thus, the optical contrast C of the known information carrier is (72-38)/72=47%. The measured optical contrast and hence the modulation which, as described above, is lower than the optical contrast do not comply with said CD-industrial standard (modulation&gt;0.6).
In the non-prepublished European Patent Application 92203773.4, filed by the Applicant of the present invention, a description is given of a manner of increasing the contrast of such a information carrier while retaining the high initial reflection, so that said information carrier can comply with the CD-industrial standard. To this end, it is proposed to provide the stack with an optically non-tight reflection layer which faces the substrate. Preferably, the stack has an MIPIM layer structure, where M is a metal layer, I a dielectric layer and P a phase-change recording layer. Viewed from the substrate, the described information carrier comprises, for example, a thin Au reflection layer, a Ta.sub.2 O.sub.5 dielectric layer, a (write once) GeTe recording layer, a Ta.sub.2 O.sub.5 dielectric layer and an optically tight Au layer. Due to interference effects, both the initial reflection and the contrast of the information carrier are increased. For said layer structure, the reflections of the unrecorded and recorded areas are 70% and 12%, respectively. Thus, the optical contrast C is 82% and hence complies with the CD-industrial standard.
A problem of the recorded information carrier which complies with the CD standard is, however, that the absorption of the laser-light beam in an unrecorded area can maximally amount to 35% due to the fact that the reflection of the unrecorded areas must be at least 65%. Consequently, the write-sensitivity of the information carrier is so small that an unusually large laser power is required for the write process.